[WIP] Feature/solverfactory

This MR mainly contains the factory implementation from !5 (closed) with some cleanup:

• Allow creating solvers / preconditioners from ParameterTrees.

• A solver (including preconditioner) can now be configured via an ini file like this:

[solver]
precond = SeqSSOR
solver = CGSolver

[SeqSSOR]
iterations = 1
relaxation = 1.8

[CGSolver]
reduction=1e-9
maxit = 5000
verbose = 3

• The corresponding dune code currently looks like this:

auto solver = SolverPrecondFactory::create<VectorType> (matrix, configuration, "solver");

• Both the actual type of the solver/preconditioner and their parameters can chosen in the ini file.
• It is also possible to create only a solver or only a preconditioner this way. See the factory doxygen for details.

This MR depends on !84 (merged).

Note: change target branch to master, once !84 (merged) is merged!

dune/istl/paamg/test/hierarchytest.cc

@@ -43,8 +43,11 @@ void testHierarchy(int N)
   typedef Dune::Amg::Hierarchy<Vector> VHierarchy;
 
   Operator op(mat, pinfo);
-  Hierarchy hierarchy(op, pinfo);
-  VHierarchy vh(b);
+  Hierarchy hierarchy(
+    Dune::stackobject_to_shared_ptr(op),
+    Dune::stackobject_to_shared_ptr(pinfo));
+  VHierarchy vh(
+    Dune::stackobject_to_shared_ptr(b));
 
   typedef Dune::Amg::CoarsenCriterion<Dune::Amg::SymmetricCriterion<BCRSMat,Dune::Amg::FirstDiagonal> >
   Criterion;